Make interactive-ness directly controllable via command line option

Make interactive-ness directly controllable via command line option

Defaults are often ok, but when they are not, people get confused.
"Why can't I kill strace?" and "Why strace dies on ^C when I want
to _tracee_ to die instead?" are typical complaints.

* strace.c: Replace 'interactive' variable with 'opt_intr' variable.
Define INTR_foo constants for its possible values.
Define 'interactive' as a macro.
(usage): Document -I n option.
(main): Parse -I n option, modify signal handling to accomidate new
-I 1 and -I 4 modes.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>